Question: Ben is $3$ times as old as Daniel and is also $4$ years older than Daniel. How old is Daniel?
Answer: We can use the given information to write down two equations that describe the ages of Ben and Daniel. Let Ben's current age be $b$ and Daniel's current age be $d$. Let Ben's current age be $b$ and Daniel's current age be $d$. ${b = 3d}$ ${b = d + 4}$ Now we have two independent equations, and we can solve for our two unknowns. Since we are looking for $d$, and both of our equations have $b$ alone on one side, this is a convenient time to use elimination. Subtracting the second equation from the first equation, we get: $0 = {3d} -{(d + 4)}$ which combines the information about $d$ from both of our original equations. Solving for $d$, we get: $2 d = 4$. $d = 2$.